Blade position detection apparatus

ABSTRACT

The front end portion of a rotational blade is accurately located in a non-contact manner although the quantity of wear of the blade has been changed. The structure is so arranged that a pair of prisms are disposed to oppose each other at a predetermined interval, either of the two prisms converges light beams emitted from a light emitting diode within a predetermined interval, and the residual prism again converges the light beams which have been diffused after they have been converged. A photodiode receives beams converged by the other prism. The disturbance light and noise are eliminated by an amplifier so as to fetch only light emitted from the light emitting diode. A blade shields light beams converged at the central portion located at a predetermined interval, while reading devices read the quantity of light received by the photodiode and as well as read the displacement of the blade. A comparator transmits a signal when the quantity of light received by said photodiode agrees with a predetermined value which has been previously stored, so that the front position of the blade is located to correspond to the wear and the like. Additionally, the opposed surfaces of the prisms are kept clean of mist and cutting dust during processing by the jetting of air across them.

BACKGROUND OF THE INVENTION Field of the Invention

Field of the Invention

The present invention relates to a blade position detection apparatus,and, more particularly, to a blade position detection apparatus fordetecting breakage or wear of a high speed rotational blade of a dicingapparatus for cutting a groove along the street of wafer at the time ofmanufacturing semiconductors.

Description of the Related Art

It is an important factor to allow the quantity of a wafer work leftfrom a groove cutting operation performed by a dicing apparatus to agreewith a predetermined value. In order to allow the quantity left from theoperation of cutting wafer to agree with a predetermined value, thelocation of the Z-axis and the repetition of the location must beperformed accurately and as well as the quantity of wear of the blademust be detected so as to be corrected.

The following two methods have been known to correct the quantity of thewear of the blade. First of all, a method has been known in which theblade is brought into contact with a machining table after apredetermined number of grooves have been cut in the wafer and thenelectricity is applied to the blade and the machining table so as tocorrect the blade. Another method has been known in which data (anempirical value) about the quantity of the wear of the blade ispreviously input as an automatic wear correction quantity to the dicingapparatus.

However, the former method encounters a problem that the blade can bedamaged because the blade is brought into contact with the machiningtable, and what is worse, another problem arises in that the machiningtable is damaged. On the other hand, the latter method raises anotherproblem in that the empirical value cannot be adapted to the variationof the quantity of the wear of the blade taken place depending upon theconditions such as the type of the wafer to be machined, the scatteringof the blades and the quantity of cutting.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a bladeposition detection apparatus capable of accurately locating the frontportion of a blade even if the quantity of wear of the blade has beenchanged due to the conditions such as the type of wafer to be machined,the scatter of the blades, and the quantity of cutting, and the like.

In order to achieve the aforesaid object, according to one aspect of theinvention, there is provided a blade position detection apparatuscomprising: light emitting means; light receiving means for receivinglight emitted by the light emitting means so as to photoelectricallyconvert received light; noise eliminating means which receives a signaltransmitted from the light receiving means, and which eliminates, fromthe signal, disturbance light except for light emitted by the lightemitting means and electric noise so as to output the signal; firstdetection means for detecting the portion of the front position of arotational blade to be inserted into a position between the lightemitting means and the light receiving means; second detection meansfor, in accordance with the output from the noise eliminating means,detecting that the rotational blade has been inserted into apredetermined position between the light emitting means and the lightreceiving means; and means for receiving the position of the frontportion of the rotational blade detected by the first detection means atthe time of the detection made by the second detection means; whereinthe front position of the rotational blade is located with respect to asubject to be machined in accordance with the received position of thefront portion of the rotational blade.

According to the present invention, the emitting surface and theincidental surface are disposed to oppose each other at a predeterminedinterval. The light emitting means emits light from the emittingsurface. The light receiving means receives light made incident on theincidental surface so as to photoelectrically convert it. Furthermore,means for eliminating disturbance light received by the light receivingmeans and generating noise is provided so that only light emitting fromthe light emitting means is transmitted from the light emitting means.

In order to achieve the aforesaid object, according to one aspect of theinvention, there is provided a blade position detection apparatus havinglight emitting means having a transparent emitting surface and emittinglight via the emitting surface, light receiving means having atransparent incidental surface positioned to oppose the emitting surfaceat a predetermined interval and receiving light made incident via theincidental surface so as to photoelectrically convert the light, firstdetection means for detecting the position of the front portion of arotational blade to be inserted into a position between the lightemitting means and the light receiving means, second detection meansfor, in accordance with an output from the light receiving means,detecting that the rotational blade has been inserted into apredetermined position between the light emitting means and the lightreceiving means; and means for receiving the of the front portionposition of the rotational blade detected by the first detection meansat the time of detection performed by the second detection means, sothat the front end portion of the rotational blade is located withrespect to a subject to be machined in accordance with the receivedfront position of the rotational blade, the blade position detectionapparatus comprising: means for storing voltage signal V anddisplacement Z of the rotational blade obtainable from the lightreceiving means whenever the rotational blade is moved by apredetermined quantity in a direction in which light converged within aninterval between the optical systems is shielded in a case where voltagesignal V0 obtainable from the light receiving means when the rotationalblade is positioned at a position at which light is not shieldedsubstantially agrees with voltage signal V1 when the emitting surfaceand the incidental surface, which have been previously stored, are keptclean; means for obtaining the relationship between the stored voltagesignal V and the displacement Z of the rotational blade; and means fordiscriminating a fact that the emitting surface and the incidentalsurface are clean in a case where the obtained relationshipsubstantially agrees with the relationship between voltage signal V andposition Z of the rotational blade realized when the emitting surfaceand the incidental surface are clean, wherein the received position ofthe front end of the rotational blade is effective.

According to the present invention, in a case where voltage signal V0obtainable from the light receiving means when the rotational blade ispositioned at the position at which it does not shield lightsubstantially agrees with voltage signal V1 which has been stored andwhich is obtainable when the emitting surface and the incidental surfaceare kept clean, the storage means stores the voltage signal V obtainedfrom the light receiving means whenever the rotational blade is moved bya predetermined quantity in a direction in which light beams convergedwithin the interval between the optical systems is shielded and thedisplacement Z of the rotational blade. The discrimination meansdiscriminates that the emitting surface and the incidental surface areclean when the relationship between the stored voltage signal V and thedisplacement Z of the rotational blade obtained by the means forobtaining the aforesaid relationship substantially agrees with therelationship between the voltage signal v and the position Z of therotational blade when the emitting surface and the incidental surfaceare clean. As a result, the received position of the front portion ofthe rotational blade is made to be effective.

In order to achieve the aforesaid objects, according to the presentinvention, there is provided a blade position detection apparatus havinglight emitting means having a transparent emitting surface and emittinglight via the emitting surface, light receiving means having atransparent incidental surface positioned to oppose the emitting surfaceat a predetermined interval and receiving light made incident via theincidental surface so as to photoelectrically convert the light, firstdetection means for detecting the front portion of a rotational blade tobe inserted into a position between the light emitting means and thelight receiving means, second detection means for, in accordance with anoutput from the light receiving means, detecting that the rotationalblade has been inserted into a predetermined position between the lightemitting means and the light receiving means; and means for receivingthe position of the front portion of the rotational blade detected bythe first detection means at the time of detection performed by thesecond detection means, so that the front end portion of the rotationalblade is located with respect to a subject to be machined in accordancewith the received position of the front portion of the rotational blade,the blade position detection apparatus comprising: washing means forjetting water and air to the emitting surface and the incidentalsurface, wherein the washing means jets water at the time of machiningso as to form a water film on the emitting surface and the incidentalsurface, so that an adhesion of cutting dust generated at the time ofmachining is prevented, and jets air at the time of detecting theposition of the rotational blade so as to remove water droplets allowedto adhere to the emitting surface and the incidental surface.

According to the present invention, the washing means is able to preventthe problem that mist including cutting dust generated during the workmachining process adheres to the emitting surface and the incidentalsurface. Therefor, the emitting surface and the incidental surface canbe kept clean, so that light quantity introduced to the light receivingmeans can be maintained at a constant quantity.

BRIEF DESCRIPTION OF THE DRAWINGS

The exact nature of this invention, as well as other objects andadvantages thereof, will be readily apparent from consideration of thefollowing specification relating to the accompanying drawings, in whichlike reference characters designate the same or similar parts throughoutthe figures thereof and wherein:

FIG. 1 is a schematic view which illustrates a blade position detectionapparatus according to the present invention;

FIGS. 2A through 2A illustrates states of operations of the elements ofthe blade position detection apparatus according to the presentinvention;

FIG. 3 is a perspective view which illustrates a state where wafer ismachined by a dicing apparatus with the blade position detectionapparatus;

FIG. 4 is an enlarged view which illustrates an essential portion of theblade position detection apparatus according to the present invention;

FIG. 5 is a plan view which illustrates a state where opposite surfacesof a pair of optical prisms of the blade position detection apparatusaccording to the present invention are watered;

FIG. 5 is a perspective view which illustrates a state where the bladeof the dicing apparatus is being detected by the blade positiondetection apparatus according to the present invention;

FIG. 7 is a flow chart which illustrates the operational state of theblade position detection apparatus according to the present invention;

FIG. 8 is a graph which illustrates results of measurements performed ina case where an optical system of the blade position detection apparatusaccording to the present invention is not contaminated;

FIG. 9 is a graph which illustrates results of measurements performed ina case where an optical system of the blade position detection apparatusaccording to the present invention is contaminated non-uniformly; and

FIG. 10 is a graph which illustrates results of measurements performedin a case where an optical system of the blade position detectionapparatus according to the present invention is contaminatednon-uniformly.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Detailed description will hereunder be given of the preferred embodimentof a blade position detection apparatus according to the presentinvention with reference to the accompanying drawings.

As shown in FIG. 1, a clock 12 of a blade position detection apparatus10 turns on a transistor 14 when the level of the clock 12 is low, sothat a light emitting diode 16 is turned on, and simultaneously, a firstFET (field effect transistor) 18 is switched off and a second FET 20 isswitched on. On the contrary, the clock 12 turns off the transistor 14when the level of the clock 12 is high, so that the light emitting diode16 is turned off, and simultaneously, the FET 18 is turned on and asecond FET 20 is turned off. As a result, the light emitting diode 16 isturned on and off at the period of the clock 12 so as to alternatelyswitch the case between a case where the output from an ope-amplifier 22to be described later is connected to an amplifier 24 and a case wherethe same is connected to an amplifier 26.

An end portion of an optical fiber 28 is disposed adjacent to the lightemitting diode 16, while another end of the same is connected to aninspection table 30. The inspection table 30 is disposed on a base 31,the inspection table 30 having a lens system 34 above the other endportion of the optical fiber 28. Furthermore, prisms 36 and 38 aredisposed on the upper surface of the inspection table

The prism 36 is disposed above the lens system 34, while a lens system40 is disposed below the prism 38. Furthermore, an end portion of anoptical fiber 32 is disposed below the lens system 40, while another endportion of the same is disposed adjacent to a photodiode 23 of theaforesaid ope-amplifier 22. Hence, light emitted from the light emittingdiode 16 is incident on the lens system 34 via the optical fiber 28.Light refracted by the lens system 34 is reflected by the prism 36 andis condensed in the prisms 36 and 38. Condensed light is reflected bythe prism 38 before it is refracted by the lens system 40. Then,refracted light is incident on the end portion of the optical fiber 32.The photodiode 23 of the ope-amplifier 22 is irradiated with light whichhas been incident on the end portion of the optical fiber 32.Simultaneously, disturbance light reaches the photodiode 23.

The negative input of the ope-amplifier 22 receives a signal which hasbeen photoelectrically converted by the photodiode 23. The aforesaidsignal includes an electric signal of light emitted from the lightemitting diode 16 and an electric signal of disturbance light. On theother hand, the positive input of the ope-amplifier 22 is supplied withoffset voltage from a mirror integrating circuit 43 constituted by theamplifier 24 and a capacitor 44, the offset voltage being obtained whenthe light emitting diode 16 has been turned off.

That is, when the light emitting diode 16 is turned off, onlydisturbance light is incident on the photodiode 23. The photodiode 23generates photoelectric motive force which corresponds to thedisturbance light. The photoelectric motive force is amplified by theope-amplifier 23 before it is stored in the mirror integrating circuit43 vie the FET 18. The output from the integrating circuit 43 is appliedto the positive input of the ope-amplifier 23 as offset voltage when thelight emitting diode 16 is turned on. Incidentally, the aforesaid offsetvoltage includes the electric signal of the disturbance light andelectric noise generated in the ope-amplifier 22, and the like (see FIG.2(B)).

Therefore, when the light emitting diode 16 is turned on, the FET 18 isturned off and the FET 20 is turned on. However, since the positiveinput of the ope-amplifier 22 is being applied with the offset voltage,the ope-amplifier 22 removes the disturbance light and the electricnoise and the like and transmits an electric signal of only light of thelight emitting diode 16, the electric signal being applied to acapacitor 46 via the FET 20 (see FIG. 2(C)). As described above, thecapacitor 46 stores the outputs from the ope-amplifier 22 whenever thelight emitting diode 16 is turned on. Hence, a buffer amplifier 26transmits a signal as designated by a curve shown in FIG. 2(D)).

The signal transmitted from the buffer amplifier 26 is read out via anAD converter 70 of a light quantity reading means 45A and is alsosupplied to a comparator 74. The comparator 74 subjects the signaltransmitted from the buffer amplifier 26 and a signal denoting athreshold value and transmitted from a storage circuit 47 to acomparison, and the comparator 74 transmits a signal to a one-shot 76when the two signals have agreed to each other. The one-shot 76transmits a C-SET signal in response to a signal transmitted from thecomparator 74. Incidentally, the storage circuit 47 previously storesthe threshold value which is arranged to be transmitted to thecomparator 74 via a DA converter 72.

A blade 54 is disposed above the prisms 36 and 38, the blade 54 beingrotatively fastened to a movable device 62. Since the movable device 62is able to move in the Y-axial direction and the Z-axial direction, theblade 54 is able to move in the Y-axial direction and the Z-axialdirection. The movable device 62 has a blade displacement reading device45B so as to read the Z-axial directional displacement of the blade 54.Incidentally, the X-axis directional movement is performed by the base31.

FIG. 3 illustrates a state where a wafer 52 is being machined by thedicing apparatus 50. FIG. 4 is an enlarged view which illustrates anessential portion of the blade position detection apparatus 10. As shownin FIGS. 3 and 4, a pipe 56 for supplying water or air is disposedadjacent to the inspection table 30, the pipe 56 having a nozzle 56A atthe front portion thereof. In the process of machining the wafer 52 asshown in FIG. 3, water is ejected from the nozzle 63 so as to besupplied to a position at which the wafer 52 is being machined by theblade 54. As a result, the blade 54 is cooled down and the cuttingfacility can be maintained.

Since the blade 54 is rotated at high speed during the machiningoperation, cutting dust of the wafer 52 generated due to the machiningis formed into mist which is present around the machining position.Therefore, water is ejected from the nozzle 56A via the pipe 56 of theblade position detection apparatus 10 in this case so as to form a waterfilm on opposing surfaces 36A and 38A of the prisms 36 and 38 (see FIG.5). As a result, the mist does not adhere to the opposing surfaces 36Aand 38A of the prisms 36 and 38.

After a predetermined number of wafers 52 have been machined, air isejected from the nozzle 56A for several seconds so as to blow theopposing surfaces 36A and 38A of the prisms 36 and 38 with air beforethe blade 54 is moved to the X-, Y- and Z-axial directions so as to bepositioned between the prisms 36 and 38 of the blade position detectionapparatus 10 (see FIG. 6). As a result, water droplet left on theopposing surfaces 36A and 38A of the prisms 36 and 38 are removed sothat the opposing surfaces 36A and 38A of the prisms 36 and 38 arecleaned up. Hence, when light to be introduced from the prism 36 intothe prism 38 passes through the opposing surfaces 36A and 38A of theprisms 36 and 38, light cannot be refracted or scattered by the waterdroplet.

A sequence so made that a water ejection mode and an air ejection modeare automatically changed over may be employed. As an alternative tothis, the ejection operation may be manually performed.

Incidentally, reference numerals 60 shown in FIGS. 1, 3 and 6 representa wafer table.

Then, the operation of the thus constituted blade position detectionapparatus according to the present invention will now be described withreference to a flow chart shown in FIG. 7.

During the period in which the wafer is being machined, water is ejectedfrom the nozzle 63 toward the machining position at which the blade 54machines the wafer 52. After a predetermined number of wafers 52 havebeen machined, the ejection of water from the nozzle 56A is stoppedbefore the blade 54 is positioned between the prisms 36 and 38 of theblade position detection apparatus 10. Then, air is jetted for severalseconds from the nozzle 56A so as to blow the opposing surfaces 36A and38A of the prisms 36 and 38 with air (step 80). Then, the blade 54 ismoved in the Y- and Z-axial directions and as well as the base 31 ismoved in the X-axial direction so that the blade 54 is set to theposition at which it is inspected by the prisms 36 and 38 (step 82).Then, the light emitting diode 16 is alternately switched in response tothe clock 12 between a state where the same is turned on and a statewhere the same is turned off, so that light quantity data transmittedfrom the amplifier 26 is read via the light quantity reading device 45A.Then, a fact that the light quantity data, which has been read, iscontinued for a predetermined time is waited for (step 84).

In this case, the ope-amplifier 22 removes the disturbance light and theelectric noise and the like, and transmits the electric signal aboutonly the light emitted from the light emitting diode 16.

After the light quantity data has been stabilized, a discrimination ismade as to whether or not the read light quantity data exceeds apredetermined value (that is, light quantity data V1 which is obtainablewhen the prisms 36 and 38 are clean enough to inspect the blade 54)(step 86). Then, the thus read light quantity data is, as initial lightquantity V0, recorded to a memory (step 88).

Then, a value of (V0/2) is set to the storage circuit 47 as thethreshold value (step 90). After setting has been completed, the blade54 is moved downwards by a degree corresponding to one pulse (the sameis downwards moved in the direction Z), and the quantity of thedisplacement of the blade 54 is read by the blade displacement readingdevice 45B (step 92). Furthermore, light quantity data at the aforesaidposition is read (step 94). After read light quantity data has beenrecorded to the memory (step 96), 1 is added to a memory address (step100).

Then, the operations in step 92 to step 106 are repeated until theoverall sampling area (that is, the area irradiated with the light beamsreflected by the prism 36) 102 shown in FIG. 6 is shielded from light bythe blade 54.

During the aforesaid repetition, the light quantity data startsdecreasing at the position at which the blade 54 is downwards moved andreaches the sampling area 102. Then, when the light quantity data hasagreed with the threshold value, the C-SET signal is transmitted fromthe one-shot 76 via the comparator 74 shown in FIG. 1. Assuming that theposition of the blade 54 at the time of the transmission of the C-SETsignal is received in a receiving means and the aforesaid position isset to the reference position Zc for the blade 54, the relativedifference Zd from surface position Zt of the work table 60, which hasbeen previously obtained, is stored (that is, Zd=|Zc-Zt|).

In a case where the surface position Zt of the aforesaid wafer table 60is obtained, the blade 54 is brought into contact with the wafer table60 so as to electrically connect the blade 54 and the wafer table 60 toeach other, and the position of the blade 54 at this time is detected sothat the position is made to be the surface position Zt. If the relativedifference Zd is once stored, position Zt (that is, Zt=|Zc'-Zd|), atwhich a new blade comes in contact with the wafer table 60, can becalculated in accordance with the stored relative difference Zd in sucha manner that reference position Zc' is detected even if the blade 54has been changed for the new blade having a different outer diameter. Asa result, the quantity of the wafer 52 left from cutting can be made tobe a predetermined value.

When the blade 54 has been further downwards moved, the overall area ofthe sampling area 102 is shielded from light and therefore the lightquantity data becomes 0. When the blade is moved downwards as describedabove, the facts that the outer diameter of the blade 54 is sufficientlylarge as compared with the diameter of the sampling area 102 and thatthe opposing surfaces of the prisms 36 and 38 are kept clean will causethe position of Zc to be positioned between a position (Za) at which thelight quantity data starts decreasing and a position (Zb) at which thelight quantity data becomes 0 (see FIG. 8). That is a relationshipexpressed by (Za+Zb)/2≈Zc is held.

The relationship between the values obtained by dividing eachdifferential value (ΔV/ΔZ), which has been obtained by differentiatingthe aforesaid curve (V-Z), by (the maximum differential value) and theposition Z of the blade 54 substantially agrees with the shape of acurve (ΔV/ΔZ-Z), which has been previously input, the shape being ashape realized when the prisms 36 and 38 are clean. In this case, adiscrimination can be made that the prisms 36 and 38 are kept clean(that is, they are in a range in which a satisfactory measuring accuracycan be obtained). Therefore, the position Zc of the blade 54 at the timeof the transmission of the C-SET signal is set to the position at whichthe blade 54 cuts the wafer, the blade is moved on the wafer 52 to reachthe reference position, and the surface position Zt of the wafer table60 is calculated in accordance with the relative difference Zd which hasbeen previously obtained. Then, the blade 54 is moved onto the wafer 52before the blade 54 is moved downwards to a position, at which apredetermined quantity of the wafer 52 left from cutting can be kept, sothat the wafer 52 is cut.

In a case where the opposing surfaces of the prisms 36 and 38 arenon-uniformly contaminated, Zc cannot be positioned between Za and Zb asshown in FIG. 9, and also the curve (ΔV/ΔZ-Z) is deviated from the shapeof the curve (ΔV/ΔZ-Z) which has been previously input. Therefore, adiscrimination is made that the coordinate position of Zc in this caseis not reliable, and an error process is performed (steps 108 and 110).

In a case where the opposing surfaces of the prisms 36 and 38 aresubstantially uniformly contaminated, Zc is positioned between Za and Zbas shown in FIG. 10, and also the curve (ΔV/ΔZ-Z) substantially agreeswith the shape of the curve (ΔV/AZ-Z) which has been previously input.Therefore, if the contamination is made substantially uniformly, andeven if the read initial light quantity V0 is lower than a predeterminedvalue (that is, light quantity data at the time when the prisms 36 and38 are kept sufficiently clean to inspect the blade 54), a certaindegree (that is, a range in which a satisfactory repetitive accuracy canbe obtained) can be brought into an allowable range. Therefore, Zc inthis case is set to be the reference position for the blade 54. However,if the read initial light quantity V0 is lower than the aforesaidpredetermined value by a certain degree, a discrimination of an error ismade in step 86.

As described above, the blade position detection apparatus according tothe present invention enables the subject machine to recognize itscondition change taken place due to the contamination of the opticalpassage of the optical sensor and the unstable state realized due to anadhesion of water droplets so that the machine can be cause to followthe changing state, or, if the satisfactory conditions cannot be met,the process can be again performed or an error process can be performed.Hence, disturbance light acting on the optical sensor or noise generatedfrom the optical sensor can be removed and therefore the sensitivity ofthe optical sensor can be maintained at a constant level. Therefore, thequantity of the wear of the blade can be accurately measured, causingthe front portion of the blade to be positioned accurately.

Although the aforesaid embodiment is so arranged that the thresholdvalue is set to the initial light quantity V0/2, another value may beset to be the threshold value.

Although the aforesaid embodiment is so arranged that the space isreduced by using the prisms 36 and 38 to constitute the blade positiondetection apparatus 10, the present invention is not limited to this.For example, the prisms 36 and 38 may be omitted from the structure.

Incidentally, the blade position detection apparatus according to thepresent invention enables a confirmation to be made as to whether or notthe spot diameter of the light beam has been contracted to a degree withwhich the optical system is able to obtain a satisfactory repetitiveaccuracy by checking the difference ΔZa-Zb| between the size Za of thesampling area of the converged light beams 102 and the size Zb of themeasured light beams 102.

As described above, according to the blade position detection apparatusand a method of discriminating the reliability of it, the displacementposition of the rotational blade at the time of the transmission of asignal made by the means for transmitting the signal is set to be thecorrection position determined by taking the wear or the like of therotational blade into consideration. Therefore, in a case where thequantity of the wear of the blade has been changed depending upon theconditions such as the type of the wafer to be machined, the scatter ofthe blades, and the quantity of cutting and the like, the rotationalblade can be located in a non-contact manner, that is, the front portionof the rotational blade can be accurately located while eliminating anecessity of bringing the blade into contact with the machining table.

Furthermore, the disturbance light and the noise can be removed, so thatthe accuracy of locating the blade can be improved.

In addition, the blade position detection apparatus according to thepresent invention is able to cause the mist including cutting dustgenerated during the process of machining the wafer not to adhere to theopposing surfaces of two optical system. Therefore, the opposingsurfaces of the optical systems can be kept clean so that the lightquantity introduced from the optical system of a pair of the opticalsystems to the light receiving means via the opposite surface of theresidual optical system can be maintained at a constant value.

Therefore, in a case where the quantity of the wear of the rotationalblade has been changed depending upon the conditions such as the type ofthe wafer to be machined, the scatter of the rotational blades, and thequantity of cutting and the like, the rotational blade can be located ina non-contact manner, that is, the front portion of the rotational bladecan be accurately located while eliminating a necessity of bringing therotational blade into contact with the machining table.

It should be understood, however, that there is no intention to limitthe invention to the specific forms disclosed, but on the contrary, theinvention is to cover all modifications, alternate constructions andequivalents falling within the spirit and scope of the invention asexpressed in the appended claims.

I claim:
 1. A blade position detection apparatus comprising:lightemitting means; light receiving means for receiving light emitted bysaid light emitting means so as to photoelectrically convert receivedlight; noise eliminating means which receives a signal transmitted fromsaid light receiving means, and which eliminates, from said signal,disturbance light except for light emitted by said light emitting meansand electric noise so as to output said signal; first detection meansfor detecting the position of a front portion of a rotational blade tobe inserted into a position between said light emitting means and saidlight receiving means; second detection means for detecting insertion ofsaid rotational blade into a predetermined position between said lightemitting means and said light receiving means in response to said signaloutput by said noise eliminating means; and means for receiving theposition of said front portion rotational blade detected by said firstdetection means at the time of the detection made by said seconddetection means; and means for locating said front portion of saidrotational blade with respect to a subject to be machined in accordancewith said received position of said front portion of the rotationalblade.
 2. A blade position detection apparatus according to claim 1,wherein said light emitting means is composed of a light emitting diodeand a first optical system for forming light emitted from said lightemitting diode into a predetermined spot light.
 3. A blade positiondetection apparatus according to claim 2, wherein said light receivingmeans is composed of a photodiode and a second optical system forcausing said predetermined spot light to be incident on said photodiode.4. A blade position detection apparatus according to claim 1, whereinsaid noise eliminating means is composed of means for turning on and offsaid light emitting means at a predetermined cycle, means which receivesa signal transmitted from said light receiving means when said lightemitting means is turned off and with which offset voltage is obtained,and means for fetching only a signal component, from which said offsetvoltage has been removed, from said signal transmitted from said lightreceiving means when said light emitting means is turned on.
 5. A bladeposition detection apparatus having light emitting means having atransparent emitting surface and emitting light via said emittingsurface, light receiving means having a transparent incidental surfacepositioned to oppose said emitting surface at a predetermined intervaland receiving light made incident via said incidental surface so as tophotoelectrically convert said light, first detection means fordetecting a front portion of a rotational blade to be inserted into aposition between said light emitting means and said light receivingmeans, second detection means for detecting that said rotational bladehas been inserted into a predetermined position between said lightemitting means and said light receiving means in response to an outputfrom said light receiving means; and means for receiving said positionof the front portion of said rotational blade detected by said firstdetection means at the time of detection performed by said seconddetection means, so that said front portion of said rotational blade islocated with respect to a subject to be machined in accordance with saidreceived position of the front portion of said rotational blade,saidblade position detection apparatus comprising: means for storing avoltage signal V and a displacement Z of said rotational bladeobtainable from said light receiving means whenever said rotationalblade is moved by a predetermined quantity in a direction in which lightconverged within an interval between said light emitting means and sidelight receiving means is shielded in a case where a voltage signal V0obtainable from said light receiving means when said rotational blade ispositioned at a position at which light is not shielded substantiallyagrees with a voltage signal V1 when said emitting surface and saidincidental surface, which have been previously stored, are kept clean;means for obtaining the relationship between said stored voltage signalV and said displacement Z of said rotational blade; and means fordiscriminating that said emitting surface and said incidental surfaceare clean in a case where said obtained relationship substantiallyagrees with the relationship between voltage signal V and position Z ofsaid rotational blade realized when said emitting surface and saidincidental surface are clean, wherein the front portion of therotational blade is located in accordance with the said receivedposition of said front portion of said rotational blade only when saidemitting surface and said incidental surface are determined to be cleanby said means for discriminating.
 6. A blade position detectionapparatus according to claim 5, wherein said second detection means hasstorage means for storing the half value of a voltage signal asthreshold value V0/2 assuming that said voltage signal obtainable fromsaid light receiving means when said rotational blade is positioned at aposition at which light is shielded, is V0, so that the moment at whichsaid voltage signal obtainable from said light receiving means agreeswith said threshold value V0/2 is detected.
 7. A blade positiondetection apparatus according to claim 5, wherein the relationshipbetween said stored light quantity V received by said light receivingmeans and said displacement Z of said rotational blade can be expressedby differential value ΔV/ΔZ and said displacement Z of said rotationalblade.
 8. A blade position detection apparatus according to claim 5further comprising noise eliminating means which receives a signaltransmitted from said light receiving means, and which eliminates, fromsaid signal, disturbance light except for light emitted from said lightemitting means and electric noise so as to transmit said signal.
 9. Ablade position detection apparatus having light emitting means having atransparent emitting surface and emitting light via said emittingsurface, light receiving means having a transparent incidental surfacepositioned to oppose said emitting surface at a predetermined intervaland receiving light made incident via said incidental surface so as tophotoelectrically convert said light, first detection means fordetecting a front portion of a rotational blade to be inserted into aposition between said light emitting means and said light receivingmeans, second detection means for detecting that said rotational bladehas been inserted into a predetermined position between said lightemitting means and said light receiving means in response to an outputfrom said light receiving means; and means for receiving said positionof the front portion of said rotational blade detected by said firstdetection means at the time of the detection performed by said seconddetention means, so that said front portion of said rotational blade islocated with respect to a subject to be machined in accordance with saidreceived position of the front portion of said rotational blade, saidblade position detection apparatus comprising:washing means for jettingwater and air to said emitting surface and said incidental surface,wherein said washing means jets water at the time of machining so as toform a water film on said emitting surface and said incidental surface,so that an adhesion of cutting dust generated at the time of machiningis prevented, and jets air at the time of detecting the position of saidfront portion of the rotational blade so as to remove water dropletsadhering to the emitting surface and said incidental surface.
 10. Ablade position detection apparatus according to claim 9, wherein saidwashing means automatically switches between a water jetting mode to beperformed at the time of machining and an air jetting mode to beperformed at the time of detecting said position of the front portion ofsaid rotational blade.